1. Field of the Invention
The present invention relates to a method and apparatus for runnerless injection molding of a synthetic resin, provided with a novel valve gate for performing intermittent temperature control to forcibly change the property of a molten or solidified resin material adjacent to the gate, capable of performing a precision molding operation without leaving an improper gate mark.
2. Description of the Prior Art
FIGS. 5 and 6 illustrate two conventional runnerless injection molding apparatus.
In the drawings, a reference numeral 1 denotes a gate, 2 a mold cavity, and 3 a runner. A valve pin 4 is enclosed within the runner 3. The tip 4a of the valve pin 4 is engageable with the gate rand 1a to close the gate 1. The valve pin 4 of valve means 5 is adapted to reciprocally move in its axial direction.
As shown in FIG. 5, the conventional apparatus is provided with a valve assembly, which is capable of opening or closing the valve by clamping and releasing the tip 4a of the valve pin 4 with respect to the gate 1, through the reciprocal motion of the piston body 7 by way of a fluid pressure control.
Another conventional valve assembly shown in FIG. 6 comprises a piston body 11 which is biased toward the gate side by a coiled spring 10 in a hollow chamber 9 of an enlarged member 8 provided in a runner 3. Therefore, a valve pin normally has its end engaged with the internal end of a gate 1 to close the gate 1, as urged by the coil spring 10.
As the injection molding operation is started, the valve pin 4, which is engaged with the gate 1, is upwardly raised by a predetermined injection pressure against the coil spring 10. After the operation is completed, the injection pressure reduces and gate pin 4 again engages at its end with the gate 11 to automatically close the valve. In the drawings, the apparatus includes a body heater for externally heating the runner assembly 3, a temperature sensor 13 for detecting the internal temperature in the runner 3, a manifold 14, and wiring 15.
In the above-described molding method, the temperature in the vicinity of the gate is not controlled, but is inevitable to be allowed to the ambient temperature of the mold. Accordingly, valve pin 4 is tend to be heated to a high temperature because of "less cooling of the gate", resulting in various problems of external appearance or mechanical strength of the molded products. On the other hand, because of "less heating of the gate", the temperature drop of the mold due to heat radiation often causes the ununiformity of resin viscosity and degradation of the products, and also causes defect operation or damage of the valve pin. Further defects are listed below:
1. Shrinkage, furrows or stringing of the products due to less cooling of the valve pin; PA1 2. Rising at the gate portion due to solidification of resin at the gate to retard the proceeding of the valve pin; PA1 3. The more the radius of the gate increases, the larger radius for the valve pin is required; PA1 4. Gate seal becomes insatisfactory due to unfused or solidified portion of resin which is produced at the gate rand, because of less temperature rise at the gate when using a high viscous resin such as polycarbonate resin; and PA1 5. Difficulty of molding when using the resins for which a fine adjustment of temperature in the range of the gate is required, such as polyamide or PET resin.